Screening apparatus in which slurries are flowed by gravity over surfaces of inclined static or stationary flat or concavely curved, so-called "sieve bend" screens, are widely used in industry for both dewatering and classifying or particle separating, with the usual preference, for minimizing blinding, a screen having a flow surface formed by bases of laterally spaced, inverted triangular or wedge-shaped screen rods or wires disposed across or perpendicular to the flow. While recognizing in particle separation that the size of the particles passed by slots of a given width between the screen rods will vary inversely with the inclination or slope of the screen, in designing such screens the customary practice is to space the screen rods uniformly over the length of the screen.
An exception to the above practice is disclosed in Detcher U.S. Pat. No. 4,113,626, in which the screens are semi-parabolic and fabricated by securing the transverse screen rods to initially straight or flat, longitudinally extending support or tie rods and then bending the assembly longitudinally of the support rods to the desired parabolic shape. The result is a screen in which the spacing between the screen rods varies over the length of the screen with the spacing at minimum over the end area of minimum radius and increasing with increase in radius toward the parabolically shaped screen's opposite end. According to Detcher, his screens are reversible and purportedly effective in dewatering regardless of whether the end of minimum or maximum radius is the leading end but the patent is silent as to the effect of the variation in slot width on particle separation and only mentions that the slots ordinarily will be of the order of 0.005 to 0.040 inches (0.127 to 1.016 mm.) wide. It is to a static screen of the spaced transverse screen rod type of improved efficiency in particle separation that the present invention is particularly directed.